1. Field of the Invention
The invention relates to a process for producing steel in a converter wherein the starting materials comprise pig iron and ferrous scrap and in which oxygen is blown into the molten metal bath in the converter.
2. Description of the Prior Art
As is known, the converter steel production process is governed by a number of chemical reactions, such as the combustion of carbon (partly into CO and partly into CO.sub.2), the combustion of phosphorus, silicon and manganese, and the combustion of iron. By adding calcareous slag to the converter the oxides formed and also sulphur can be separated from the steel bath and absorbed into the layer of slag.
The quantity of scrap which can be included in the contents of the converter is determined by the heat balance of the various reactions occurring and the required tapping temperature. As a rule, scrap is considerably cheaper than molten pig iron, and it is therefore usually desirable, in order to achieve a low cost price, to maximize the use of scrap.
Although the combustion of phosphorus, silicon, manganese and iron contribute to the heat balance, and the presence of the oxides formed in the slag does have certain advantages, it is advisable for economic reasons to limit the combustion of manganese and iron. Among other things because of the rapidity of the converter process, the so-called dynamic equilibrium, which arises in practice, of the various elements present in the steel bath and the slag is considerably more unfavourable than the theoretical thermodynamic equilibrium. Attempts have therefore been made to find a method in which the oxidation of manganese and iron is limited and the use of scrap is increased.
By bubbling inert gas through the bath from the bottom of the converter, a more homogeneous steel bath can be obtained, which appears to result in a lower absorption of iron into the slag and in a reduction in the combustion of manganese. Particularly for pig iron with a low phosphorus content, it has however been found that this bubbling technique can indeed give reduced combustion of iron and manganese but in many cases a reduced use of scrap has to be accepted in order to reach the required tapping temperature. Dutch patent application No. 8105221 (European patent application No. 82201426) describes a structure which makes it possible to pass inert gas through a porous bottom brick in the converter.
Dutch patent application No. 8104474 (European patent application No. 82201165) describes a structure for an oxygen blowing lance which has, above the main oxygen blowing head, outlets for secondary oxygen which is blown above the surface of the steel bath. It is known that by means of this secondary oxygen more of the CO formed from the bath can be converted to CO.sub.2, which contributes to the heat balance with the consequence of a possible increase in the amount of scrap used.
It has also been suggested that during the blowing of oxygen a quantity of carbon-containing material is added to the reaction, to serve as fuel and thus raise the bath temperature. In general however it has been found that a carbon-containing fuel added in this way is burned with very poor thermal efficiency.